The present disclosure relates generally to downhole well-logging tools and, more particularly, to sourceless X-ray downhole well-logging tools free of radioisotopic reference sources.
This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present techniques, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
Many downhole well-logging tools use emissions of radiation, such as gamma-rays, to determine a property of a subterranean formation such as lithology or density. Once emitted into the formation, the gamma-rays may interact with the formation through Compton scattering, which may attenuate the gamma-rays, and/or the photoelectric effect, which may absorb the gamma-rays. The degree to which the formation causes the gamma-rays to be Compton scattered and/or to be absorbed via the photoelectric effect may depend respectively on the density and lithology of the formation. That is, formations of various densities and lithologies will cause Compton scattering and absorption via the photoelectric effect in a predictable manner. Thus, by detecting the spectrum of gamma-rays that return to the downhole tool, the density and/or lithology of the formation may be determined.
Conventionally, such a downhole tool may emit radiation into the formation using a radioisotopic gamma-ray source, such as 137Cs. In addition, other radioisotopic gamma-ray sources of relatively low-strength (e.g., a 137Cs source of approximately 0.5-10 μCi), referred to as reference sources or stabilization sources, may be located near the radiation detectors of the downhole tool. These reference sources may emit a known amount of radiation directly at the radiation detectors. Because the sensitivity of the radiation detectors may vary and also may depend on environmental factors, which may change greatly as the downhole tool travels through the formation, the gain of the radiation detectors may be stabilized based on the radiation emitted by the reference sources. Thus, when the gamma-rays that return to the downhole tool after interacting with the formation are detected by the radiation detectors, the radiation detectors may have stable gains despite unique sensitivities or changing environmental conditions.
The use of radioisotopic sources may have several drawbacks. For example, radioisotopic sources in downhole tools may require special handling when the radioisotopic sources are inserted into or removed from the downhole tool. Additionally, these radioisotopic sources may require additional shielding during transportation and storage, as well as additional security during such transportation and storage. Indeed, in many countries, even very-low-strength radiation sources (e.g., 10−6 Ci) may be considered radioisotopic sources subject to burdensome regulations.